1. Field of the Invention
This invention relates generally to diode pumped lasers, and more particularly to lasers that use a diode source to pump a gain medium that provides a strong thermal lens.
2. Description of Related Art
Diode lasers, either as single spatial mode diodes, diode arrays, or diode bars, have been utilized to pump a variety of gain mediums. Early work by Baer et al. determined that the efficiencies of array-pumped lasers are greatly enhanced by an end-pumped design that focusses the multimode diode pump light to a diameter in the gain medium that is smaller than the TEM.sub.00 mode diameter. It is also important that the absorption length of the diode pump light in the gain medium is sufficiently short to confine most of the multimode diode pump light to within the TEM.sub.00 mode volume. See for example, U.S. Pat. Nos. 4,653,056 and 4,656,635. This classic invention has been called "mode-matching".
The effect of mode-matching is to maximize the coupling between the TEM.sub.00 mode of the laser resonator and the excited volume in the gain medium within the resonator. In turn, the optical slope efficiency and the overall optical efficiency are both maximized for a given pump source. In a classic mode-matched geometry, the ratio R of the TEM.sub.00 mode diameter to the pump beam diameter in a diode-end-pumped Nd:YAG laser is usually greater than 1, for example 1.3. This is typically accomplished in a longitudinally-pumped or end-pumped configuration, where the propagation direction of the diode pump beam is nominally parallel to that of the eigenmode within the gain medium of the diode-pumped laser. At lower diode pump powers, such as 2 W, mode-matching has proven to be very successful in achieving the required efficiencies and in producing gaussian beams that are very nearly diffraction limited. Typically, at these pump powers and with the typical pump spot sizes that are used in end-pumped lasers that utilize diode arrays, thermal focussing, lensing, or thermal aberrations are not significant enough to degrade the efficiency of the end-pumped, mode-matched system. It is also known in the art that the exact shape of the pump light distribution does not matter in these mode-matched, low power systems. The multimode, lobed pattern from a diode array still induces TEM.sub.00 operation if end-pumping an Nd:YAG laser in a mode-matched configuration.
At low pump powers, longitudinal mode-matching techniques work well both for materials with strong thermal focussing characteristics (like Nd:YAG and Nd:YVO.sub.4), and for materials with weak thermal focussing characteristics (like Nd:YLF). This is because pump powers below about 2 W with typical pump spot sizes result in pump power densities that are typically too low to induce a thermal lens of a magnitude that could significantly alter the properties of a typical diode-pumped laser resonator. These power levels are available from diode arrays. Recently, higher power arrays have become available (.about.4 W, for example, from Spectra-Diode Labs). At still higher pump powers, such as those available from 10 and 20 W diode laser bars, thermal effects become significant in end-pumped lasers, and must be taken into account. Still, end-pumping remains attractive because of the potential of a highly efficient diode-pumped laser system.
At the higher pump powers of diode bars, thermal lenses of appreciable focussing power can be generated in the gain medium by the diode pump light, especially when longitudinal pumping is employed. The aberrations that are inherently associated with these strong thermal lenses are thought to limit the efficiency of high power diode-pumped lasers. See for example S. C. Tidwell, J. F. Seamans, M. S. Bowers, A. K. Cousins, IEEE J. Quantum Electron. 24, 997 (1992).
"Strong" and "weak" thermal focussing or lensing can be defined as follows. When a "strong thermal lens" is generated, the focussing power of the pump-induced thermal lens is at least comparable to that of the other optics in the laser resonator. A strong thermal lens significantly changes the size and divergence of a laser resonator eigenmode within the laser resonator. With a "weak thermal lens", the focussing power of the pump induced lens is substantially lower than that of the other optics in the laser resonator, such as mirrors and typical lenses. The thermal lens can be considered weak if the other optics in the laser resonator dictate the size and divergence of the resonator eigenmode, while the thermal lens has little effect on the eigenmode properties. Typically, materials that exhibit strong thermal focussing or strong thermal lensing also exhibit strong aberrations, which have been detrimental to laser performance of the prior art.
Materials that exhibit strong thermal focussing can have other properties that make them suitable or desirable for certain applications. With respect to Nd:YLF, for example, Nd:YVO.sub.4 exhibits high gain and a short upper state lifetime. Nd:YAG has an intermediate gain and an intermediate upper state lifetime. These properties provide important adjustable parameters when designing a Q-switched laser with high pulse energy or high repetition rate, or a laser that is insensitive to optical feedback. Additionally, certain properties of Nd:YVO.sub.4 make it attractive for diode pumping; its absorption coefficient at the diode pump wavelength of .about.809 nm is extremely high, permitting efficient coupling of diode pump light into the Nd:YVO.sub.4 gain medium.
The materials Nd:YAG, Nd:YVO.sub.4, and Nd:YLF are ready examples of materials with strong and weak thermal focussing characteristics, with Nd:YLF typically categorized as weak. Examples of materials that can exhibit strong thermal lenses are Nd:YAG and Nd:YVO.sub.4. Pump-induced surface distortion can contribute to the thermal lens magnitude, but the effect in Nd:YVO.sub.4 or Nd:YAG is primarily due to a strong dependence of the material's index of refraction upon the local temperature (dn/dT) in the material. While this dependence is about one order of magnitude smaller for Nd:YLF that it is for Nd:YAG and Nd:YVO.sub.4, it should be noted that even the focussing, or defocussing, power of a thermal lens in a material like Nd:YLF must be considered in a laser resonator if its focussing power is comparable to that of the other intracavity optics. As an example of a weak thermal lens, it is usually possible to design a laser resonator using Nd:YLF in a way that results in a thermal lens of focussing power that is weaker than that of other intracavity optics. The aberrations in the thermal lens of Nd:YLF in this type of laser are also typically weak.
It is important to note that a diode-pump-induced thermal lens is not a perfect lens, but is rather an aberrated lens. In a typical high power diode-pumped laser design, a strong thermal lens is inherently an aberrated thermal lens. It is thought that the aberrations in the strong pump induced thermal lenses limit the efficiency of high power diode bar pumped lasers. See for example, S. C. Tidwell, J. F. Seamans, M. S. Bowers, A. K. Cousins, IEEE J. Quantum Electron. 24, 997 (1992). This is because the thermally-induced aberrations add significant diffractive loss to resonators when conventional mode-matching techniques are employed. An aberrated thermal lens is one where the optical path difference (OPD) as a function of radius cannot be adequately fit by a simple parabola. A hypothetical perfect thermal lens would have an OPD profile that could be fit by a perfect parabola. For a typical aberrated thermal lens, the optical path difference as a function of radius is most nearly parabolic near its center, but deviates strongly from a parabola in its wings, as heat flows out of the pumped center into the surrounding gain medium. See for example J. Frauchiger, P. Albers, H. P. Weber, IEEE J. Quantum Electron. 24, 1046 (1992).
It has been reported that the efficiency of a laser system with aberrated thermal lensing is reduced with respect to a laser system without aberrated thermal lensing because the thermal aberration acts as a pump-power dependent loss in the laser resonator. In order to make a relative comparison to a high power, high efficiency laser, this particular type of diode-pumped laser is used as a benchmark. The reference laser is a diode-bar-pumped Nd:YLF laser, as reported by S. B. Hutchinson, T. Baer, K. Cox, P. Gooding, D. Head, J. Hobbs, M. Keirstead, and G. Kintz, Advances of 3-10 Watt Average Power Diode Pumped Lasers in Diode Pumping of Average Power Solid State Lasers, G. F. Albrecht, R. J. Beach, S. P. Velsko, Editors, Proc. SPIE 1865, 61-72. The authors reported a diode-bar-pumped Nd:YLF laser that was designed in a way that the thermal lens of the Nd:YLF could be considered weak, therefore presented only weak thermal aberrations. This laser provided 6 W of polarized output power (P.sub.o) in a TEM.sub.00 mode of M.sup.2 &lt;1.1 for 17 W of diode-bar pump power incident (P.sub.i) upon the Nd:YLF gain media. The optical efficiency (P.sub.o /P.sub.i) of this laser is .about.35%, while the optical slope efficiency (dP.sub.o /dP.sub.i) is .about.40%. This laser is a highly efficient, high power laser and can be considered a benchmark as a high power, highly efficient diode-bar-pumped laser that operates in a nearly diffraction-limited TEM.sub.00 mode. Any diode-bar-pumped laser with comparable power and optical efficiency in a nearly diffraction-limited TEM.sub.00 mode can therefore be called a highly efficient, high power, diode-bar-pumped laser.
High power diode-bar-pumped lasers that have been built using gain mediums with strong thermal focussing have been reported to be less efficient than this benchmark. For example, using the same definitions, overall optical efficiencies (P.sub.o /P.sub.i) of only about 16% have been reported for diode-bar end-pumped Nd:YAG operating in the TEM.sub.00 mode at the 6 W output level. The reported multimode efficiency achieved with Nd:YAG is higher, but multimode beams are not useful for many applications. See for example S. C. Tidwell, J. F. Seamans, M. S. Bowers, A. K. Cousins, IEEE J. Quantum Electron. 24, 997 (1992). A 26% optical efficiency (P.sub.o /P.sub.i) was reported for an Nd:YAG laser at the 60 W level, but the TEM.sub.00 laser beam quality was worse than the benchmark at M.sup.2 &lt;1.3, the beam was unpolarized, and the laser used an aspheric optic for aberration compensation that worked over only a narrow range of pump power. See for example S. C. Tidwell and J. F. Seamans, 60-W Near TEM00, cw Diode-End-Pumped Nd:YAG Laser, in Diode Pumping of Average Power Solid State Lasers, G. F. Albrecht, R. J. Beach, S. P. Velsko, Editors, Proc. SPIE 1865, 85-92, herein referred to as "Tidwell et al". One report also indicated a 36% optical efficiency (P.sub.o /P.sub.i) for a TEM.sub.00 Nd:YAG laser at the 7.6 W output level, pumped by 38 individual, polarization combined, fiber coupled diode arrays that provided an incident power of 21.1 W. A serious drawback of this system is that diode bars were not used, hence the tremendous complexity, cost, and low wallplug efficiency (P.sub.o divided by electrical input power) of 38 individual polarization combined fiber-coupled diodes. This was reported by Y. Kaneda, M. Oka, H. Masuda, and S. Kubota, 7.6 W of cw Rradiation in a TEM.sub.00 Mode From a Laser-Diode-End-Pumped Nd:YAG Laser, Opt. Lett. 17, 1003 (1992), herein referred to "Kaneda et al".
In spite of all of these difficulties, it would be useful to develop a diode-bar-pumped laser that can make use of gain medium materials with strong thermal focussing and still operate at high power with high efficiency in a nearly diffraction-limited TEM.sub.00 mode. This is because some of these materials with strong thermal focussing have other desirable properties that make them desirable for certain applications. With respect to Nd:YLF, Nd:YVO.sub.4 exhibits high gain and a short upper state lifetime. Nd:YAG has an intermediate gain and an intermediate upper state lifetime. These properties provide important adjustable parameters when designing a Q-switched laser with high pulse energy or high repetition rate, or a CW laser that is insensitive to optical feedback. Additionally, certain properties of Nd:YVO.sub.4 make it attractive for diode pumping; its absorption coefficient at the diode pump wavelength of .about.809 nm is extremely high, permitting efficient coupling of diode pump light into the Nd:YVO.sub.4 gain medium.
Materials with strong thermal focussing characteristics like Nd:YAG and Nd:YVO.sub.4 have been used in certain lasers with pump powers greater than 2 W. However, strong, aberrated thermal lenses are generated in end-pumped configurations. Additionally, strong thermal birefringence is seen in YAG. This is primarily because the index of refraction in these materials is a strong function of temperature (dn/dT is large), and the deposition of heat by the pump beam induces large thermal gradients. It has been generally believed that strong thermal focussing or lensing is a hindrance in the design and construction of an efficient laser with high beam quality, and therefore, there has not been great success in the use of materials with strong thermal focussing in highly efficient, high power diode-bar-pumped lasers.
Aberrations in the pump-induced thermal lens can be canceled or corrected with specially shaped aspheric lenses or aspheric gain medium faces within the laser cavity. Ideally, if the pump-induced aberrations are perfectly corrected, a favorable ratio R (greater than unity) of TEM.sub.00 mode diameter to pump beam diameter can be employed, and optical efficiencies can approach those of more conventional mode-matched lasers that do not have significant aberration. There are, however, some major limitations to these types of non-dynamic compensation schemes. They do not work well over a range of pump powers since the magnitude of the thermal lens, and its aberrations, is a dynamic function of the pump and intracavity powers. These designs have limited appeal because they work over a very small range of diode pump powers. However, they have been explored, (see Tidwell, et al).
When Nd:YAG is used as the laser medium, an additional problem arises in a high-power, end-pumped geometry. This effect is thermal birefringence, and is well-known in lamp-pumped Nd:YAG lasers (see for example Koechner, Solid-State Laser Engineering, vol. 3, p. 393). Many laser applications require a polarized beam; thermal birefringence results in spatially-dependent polarization rotation of parts of the eigenmode within the cavity and thus loss when the eigenmode passes through an intracavity polarizer. This loss can be significant, and can severely limit the output power of a polarized laser. See for example Kaneda et al and Tidwell et al. In some cases, when multiple Nd:YAG gain mediums are used, polarization rotation schemes can be used to induce cancellation of thermal birefringence between similarly-pumped Nd:YAG gain mediums (see Tidwell et al). However, the techniques of the prior art are imperfect, and are difficult to implement when only one gain medium is placed within the laser resonator. It would be desirable to minimize thermal birefringence in an end-pumped Nd:YAG gain medium, or in any gain medium where the magnitude of thermal birefringence is detrimental.
A need exists for highly efficient lasers. A low efficiency laser requires more diode pump power to achieve a desired laser output power. Increasing the pump power from a particular diode bar source increases the temperature of the diode junction, and the lifetime of the diode source is degraded in a predictable but highly undesirable way. This is unacceptable for applications that require long life. A low efficiency laser may require the use of additional diode sources to achieve a particular laser output power. This may be unacceptable for applications that are sensitive to cost or complexity.
A need exists for a highly efficient, high power laser design that uses materials with strong thermal focussing properties but provides high quality beams over a range of pump powers. There is a need for a solid state diode pumped laser which has reduced thermal berefringence. There is a further need for a solid state diode pumped laser with a resonator length of 1 to 10 Rayleigh ranges.